Document named captures

docs/modules/ROOT/pages/node_pattern.adoc

@@ -126,6 +126,7 @@ Ruby code with a method call with two integer literals as arguments, `foo(1, 2)`
 To match just those method calls where the first argument is a literal `1`, use `(send nil? :foo (int 1) int)`.
 Any child that is a node can be a target for nested matching.
 
+[#any-single-node]
 == `_` for any single node
 
 `_` will check if there's something present in the specific position, no matter the
@@ -314,6 +315,37 @@ The following pattern will have two captures, both arrays:
 (send nil? $int+ (send $...))
 ----
 
+Captures <<any-single-node, for any single node>> can be given a name to make them more descriptive:
+
+----
+(send nil? $_method_name)
+----
+
+You can also reference them later in the pattern to match against the value that was previously captured:
+
+----
+(pair
+  (_ $_key)
+  (_ $_key))
+----
+
+This will match hash pairs where the key is equal to the value.
+
+[source,ruby]
+----
+# Matches:
+{ a: :a } # (hash
+          #   (pair
+          #     (sym :a)
+          #     (sym :a)))
+
+# But not this:
+{ a: :b } # (hash
+          #   (pair
+          #     (sym :a)
+          #     (sym :b)))
+----
+
 == `^` for parent
 
 One may use the `^` character to check against a parent.
@@ -347,6 +379,7 @@ we can write:
 This would match both of these methods `foo` and `bar`, even though
 these `return` for `foo` and `bar` are not at the same level.
 
+[source,ruby]
 ----
 def foo              # (def :foo
   return 42          #   (args)